1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array substrate for an in-plane switching liquid crystal display (IPS LCD) device.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device has an upper substrate and a lower substrate, which are spaced apart and face each other, and a liquid crystal layer disposed between the upper and lower substrates. Each of the substrates includes an electrode and the electrodes of each substrate also face each other. The LCD device uses an optical anisotropy of liquid crystal and produces an image by controlling light transmissivity by varying the arrangement of liquid crystal molecules, which are arranged by an electric field.
As it has a high resolution and can display an excellent moving image, an LCD device, which includes thin film transistors and pixel electrodes arranged in a matrix form, is widely used. The LCD device is referred to as an active matrix liquid crystal display (AMLCD).
A related art liquid crystal display (LCD) device will be described hereinafter more in detail with reference to FIG. 1. FIG. 1 is an exploded perspective view illustrating a conventional LCD device. The conventional LCD device 11 has upper and lower substrates 5 and 22, which are spaced apart and face each other, and also has a liquid crystal layer 14 to be interposed between the upper substrate 5 and the lower substrate 22.
A gate line 13 is formed horizontally in the context of the figure on the inside of the lower substrate 22 and a data line 15 is formed vertically in the context of the figure on the inside of the lower substrate 22. The gate line 13 and the data line 15 cross each other to define a pixel area “P”. A thin film transistor “T” is situated at the crossing of the gate line 13 and the data line 15. A pixel electrode 17, which is electrically connected to the thin film transistor “T”, is formed in the pixel area “P”. The pixel electrode 17 is made of a transparent conductive material such as Indium-Tin-Oxide (ITO) or Indium Zinc Oxide (IZO).
Next, a black matrix 6, which has an opening corresponding to the pixel electrode 17, is formed on the inside of the upper substrate 5. A color filter 7 corresponding to the opening of the black matrix 6 is formed on the black matrix 6. The color filter 7 includes three colors: red (R), green (G) and blue (B). Each color corresponds to a respective pixel electrode 17. Subsequently, a transparent common electrode 18 is formed on the color filter 7.
In the related art LCD device of FIG. 1, when voltage is applied to the pixel electrode 17 and the common electrode 18, an electric field is induced between the pixel electrode 17 and the common electrode 18 in a direction perpendicular to the upper and lower substrates 5 and 22. Molecules of the liquid crystal layer 14 are arranged by the electric field and light is emitted through the arranged liquid crystal layer 14 from a back light (not shown) disposed below the conventional LCD device, so that pictures are displayed.
The conventional LCD device having the above-mentioned structure, in which the liquid crystal layer is driven by the electric field perpendicular to the upper and lower substrates, has a high transmittance and a high aperture ratio, and the common electrode of the upper substrate is grounded so that breaking of the device due to static electricity is prevented.
The conventional LCD device, generally, uses twisted nematic (TN) mode liquid crystal, the orientation of which is parallel to substrates and is continuously twisted from one substrate to another substrate by 90 degrees.
FIGS. 2A and 2B are schematic views conceptually showing operation modes of a related art twisted nematic liquid crystal display (TNLCD) device. The liquid crystal has positive dielectric anisotropy, and is arranged in parallel to the direction of the electric field.
In FIG. 2A, when voltage is not applied, orientation of the long axis of the liquid crystal layer 14 is parallel to substrates 22 and 5, and molecules of the liquid crystal layer 14 are continuously twisted from the lower substrate 22 to the upper substrate 5 by 90 degrees.
In FIG. 2B, when voltage is applied to the pixel and common electrodes 17 and 18, the electric field, which is perpendicular to the lower and upper substrates 22 and 5, is induced between the lower and upper substrate 22 and 5, and molecules of the liquid crystal layer 14 are arranged in parallel to the electric field except for molecules of the liquid crystal layer 14 close to the pixel and common electrodes 17 and 18. At this time, the liquid crystal layer, generally, has molecules twisted at an extreme value of 90 degrees.
However, the above-mentioned liquid crystal display device, in which an electric field is induced perpendicular to the substrates and which includes molecules of the liquid crystal layer parallel to the electric field, has a disadvantage of a narrow viewing angle. To overcome the narrow viewing angle, an in-plane switching (IPS) LCD device was developed. The IPS LCD device implements an electric field that is parallel to the substrates, which is different from the TNLCD device. A detailed explanation of a conventional IPS LCD device and its operation modes will be provided with reference to the following figures.
FIG. 3 is a schematic cross-sectional view of a related art in-plane switching liquid crystal display device. As shown in FIG. 3, the upper and lower substrates 32 and 30 are spaced apart from each other, and a liquid crystal layer 33 is interposed therebetween. The upper and lower substrates 32 and 30 are referred to as a color filter substrate and an array substrate, respectively. Pixel and common electrodes 34 and 36 are disposed on the lower substrate 30. The pixel and common electrodes 34 and 36 are parallel with each other and spaced apart from each other. Molecules of the liquid crystal layer 33 are aligned by a lateral electric field between the pixel and common electrodes 34 and 36.
FIGS. 4A and 4B are views illustrating operations of the liquid crystal for IPS mode at on and off states of the applied voltage. FIG. 4A conceptually illustrates “off state” operation modes for a related art IPS LCD device. In the off state, the long axis of the liquid crystal layer 33 maintains an initial arrangement, which is made by a method such as a rubbing. The pixel and common electrodes 34 and 36 are parallel with each other.
FIG. 4B conceptually illustrates “on state” operation modes for a related art IPS LCD device. In the on state, an in-plane electric field 35, which is parallel to the surface of the lower substrate, is generated between the pixel and the common electrodes 34 and 36. The pixel electrode 34 and common electrode 36 are formed together on the lower substrate for this reason. Thereby, molecules of the liquid crystal layer 33 are aligned such that the long axes thereof are parallel to the substrates and perpendicular to the pixel and common electrodes 34 and 36.
FIGS. 5A and 5B are schematic plan views conceptually showing operation modes of a related art in-plane switching liquid crystal display device. Here, arrow 41 of FIG. 5A and 41a of FIG. 5B show a direction of the long axis of the liquid crystal. In the off state of FIG. 5A, the long axis of the liquid crystal has a definite angle with respect to the pixel and common electrodes 34 and 36, and maintains the angle. On the other side, in the on state of FIG. 5B, electric field 35 is formed in a direction perpendicular to the pixel and common electrodes 34 and 36. Then, the liquid crystal (not shown) turns clockwise, so that the long axis thereof coincides with the direction of the electric field 35. Here, the extreme value of the liquid crystal in the IPS LCD device is smaller than that in the TNLCD device.
As stated above, the IPS LCD device uses the lateral electric field 35 because the pixel and common electrodes 34 and 36 are formed on the same substrate. The IPS LCD device has a wide viewing angle and low color dispersion. Specifically, the viewing angle of the IPS LCD device is about 70 degrees in the directions up, down, right, and left. In addition, the fabricating processes of this IPS LCD device are simpler than other various LCD devices.
However, the IPS LCD device has a disadvantage of color shift according to a viewing angle. FIG. 6 is a chromaticity diagram according to a viewing angle of a related art in-plane switching liquid crystal display (IPS LCD) device. In FIG. 6, standard white light has coordinate values of about 0.329 and 0.333. The color shift results from birefringence of the liquid crystal, and this is also discussed in “Advanced 18.1-inch Diagonal Super-TFT-LCDs with Mega Wide Viewing Angle and Fast Response Speed of 10 ms” of S. Endow et al., page 187, IDW 99′.
On the other hand, FIG. 7 shows a diagram of transmittance versus a viewing angle according to gray level of a related art liquid crystal display device. In FIG. 7, the gray level is divided into 8 levels. In a first level, while the transmittance at the front part, i.e. a viewing angle of 0 degrees, is zero percent, the transmittance of a viewing angle of ±60 degrees is about 20 percent, which is larger than that of a fourth level. That is, when in the black mode of the first level, black shows at the viewing angle of 0 degrees, but white shows at the viewing angle of about ±60 degrees. Therefore, gray inversion occurs.